Tamas L. Horvath

Leptin activates anorexigenic POMC neurons through a neural network in the arcuate nucleus

The administration of leptin to leptin-deficient humans, and the analogous Lepob/Lepob mice, effectively reduces hyperphagia and obesity. But common obesity is associated with elevated leptin, which suggests that obese humans are resistant to this adipocyte hormone. In addition to regulating long-term energy balance, leptin also rapidly affects neuronal activity. Proopiomelanocortin (POMC) and neuropeptide-Y types of neurons in the arcuate nucleus of the hypothalamus are both principal sites of leptin receptor expression and the source of potent neuropeptide modulators, melanocortins and neuropeptide Y, which exert opposing effects on feeding and metabolism. These neurons are therefore ideal for characterizing leptin action and the mechanism of leptin resistance; however, their diffuse distribution makes them difficult to study. Here we report electrophysiological recordings on POMC neurons, which we identified by targeted expression of green fluorescent protein in transgenic mice. Leptin increases the frequency of action potentials in the anorexigenic POMC neurons by two mechanisms: depolarization through a nonspecific cation channel; and reduced inhibition by local orexigenic neuropeptide-Y/GABA (γ-aminobutyric acid) neurons. Furthermore, we show that melanocortin peptides have an autoinhibitory effect on this circuit. On the basis of our results, we propose an integrated model of leptin action and neuronal architecture in the arcuate nucleus of the hypothalamus.

The distribution and mechanism of action of ghrelin in the CNS demonstrates a novel hypothalamic circuit regulating energy homeostasis.

The gastrointestinal peptide hormone ghrelin stimulates appetite in rodents and humans via hypothalamic actions. We discovered expression of ghrelin in a previously uncharacterized group of neurons adjacent to the third ventricle between the dorsal, ventral, paraventricular, and arcuate hypothalamic nuclei. These neurons send efferents onto key hypothalamic circuits, including those producing neuropeptide Y (NPY), Agouti-related protein (AGRP), proopiomelanocortin (POMC) products, and corticotropin-releasing hormone (CRH). Within the hypothalamus, ghrelin bound mostly on presynaptic terminals of NPY neurons. Using electrophysiological recordings, we found that ghrelin stimulated the activity of arcuate NPY neurons and mimicked the effect of NPY in the paraventricular nucleus of the hypothalamus (PVH). We propose that at these sites, release of ghrelin may stimulate the release of orexigenic peptides and neurotransmitters, thus representing a novel regulatory circuit controlling energy homeostasis.

Obesity is associated with hypothalamic injury in rodents and humans

Rodent models of obesity induced by consuming high-fat diet (HFD) are characterized by inflammation both in peripheral tissues and in hypothalamic areas critical for energy homeostasis. Here we report that unlike inflammation in peripheral tissues, which develops as a consequence of obesity, hypothalamic inflammatory signaling was evident in both rats and mice within 1 to 3 days of HFD onset, prior to substantial weight gain. Furthermore, both reactive gliosis and markers suggestive of neuron injury were evident in the hypothalamic arcuate nucleus of rats and mice within the first week of HFD feeding. Although these responses temporarily subsided, suggesting that neuroprotective mechanisms may initially limit the damage, with continued HFD feeding, inflammation and gliosis returned permanently to the mediobasal hypothalamus. Consistent with these data in rodents, we found evidence of increased gliosis in the mediobasal hypothalamus of obese humans, as assessed by MRI. These findings collectively suggest that, in both humans and rodent models, obesity is associated with neuronal injury in a brain area crucial for body weight control.

Ghrelin modulates the activity and synaptic input organization of midbrain dopamine neurons while promoting appetite

The gut hormone ghrelin targets the brain to promote food intake and adiposity. The ghrelin receptor growth hormone secretagogue 1 receptor (GHSR) is present in hypothalamic centers controlling energy metabolism as well as in the ventral tegmental area (VTA), a region important for motivational aspects of multiple behaviors, including feeding. Here we show that in mice and rats, ghrelin bound to neurons of the VTA, where it triggered increased dopamine neuronal activity, synapse formation, and dopamine turnover in the nucleus accumbens in a GHSR-dependent manner. Direct VTA administration of ghrelin also triggered feeding, while intra-VTA delivery of a selective GHSR antagonist blocked the orexigenic effect of circulating ghrelin and blunted rebound feeding following fasting. In addition, ghrelin- and GHSR-deficient mice showed attenuated feeding responses to restricted feeding schedules. Taken together, these data suggest that the mesolimbic reward circuitry is targeted by peripheral ghrelin to influence physiological mechanisms related to feeding.

Ghrelin controls hippocampal spine synapse density and memory performance

The gut hormone and neuropeptide ghrelin affects energy balance and growth hormone release through hypothalamic action that involves synaptic plasticity in the melanocortin system. Ghrelin binding is also present in other brain areas, including the telencephalon, where its function remains elusive. Here we report that circulating ghrelin enters the hippocampus and binds to neurons of the hippocampal formation, where it promotes dendritic spine synapse formation and generation of long-term potentiation. These ghrelin-induced synaptic changes are paralleled by enhanced spatial learning and memory. Targeted disruption of the gene that encodes ghrelin resulted in decreased numbers of spine synapses in the CA1 region and impaired performance of mice in behavioral memory testing, both of which were rapidly reversed by ghrelin administration. Our observations reveal an endogenous function of ghrelin that links metabolic control with higher brain functions and suggest novel therapeutic strategies to enhance learning and memory processes.

Minireview: Ghrelin and the Regulation of Energy Balance—A Hypothalamic Perspective

The recently discovered hormone, ghrelin, has been recognized as an important regulator of GH secretion and energy homeostasis. Orexigenic and adipogenic ghrelin is produced by the stomach, intestine, placenta, pituitary, and possibly in the hypothalamus. The concentration of circulating ghrelin, principally derived from the stomach, is influenced by acute and chronic changes in nutritional state. To date, most studies focused on the role of ghrelin in GH secretion or its function in complementing leptin action to prevent energy deficits. The potential significance of ghrelin in the etiology of obesity and cachexia as well as in the regulation of growth processes is the subject of ongoing discussions. A large quantity of information based on clinical trials and experimental studies with ghrelin and previously available synthetic ghrelin receptor agonists (GH secretagogues) must now be integrated with a rapidly increasing amount of data on the central regulation of metabolism and appetite. In this overview, we summarize recent findings and strategies on the integration of ghrelin into neuroendocrine networks that regulate energy homeostasis.

UCP2 mediates ghrelin’s action on NPY/AgRP neurons by lowering free radicals

The gut-derived hormone ghrelin exerts its effect on the brain by regulating neuronal activity. Ghrelin-induced feeding behaviour is controlled by arcuate nucleus neurons that co-express neuropeptide Y and agouti-related protein (NPY/AgRP neurons). However, the intracellular mechanisms triggered by ghrelin to alter NPY/AgRP neuronal activity are poorly understood. Here we show that ghrelin initiates robust changes in hypothalamic mitochondrial respiration in mice that are dependent on uncoupling protein 2 (UCP2). Activation of this mitochondrial mechanism is critical for ghrelin-induced mitochondrial proliferation and electric activation of NPY/AgRP neurons, for ghrelin-triggered synaptic plasticity of pro-opiomelanocortin-expressing neurons, and for ghrelin-induced food intake. The UCP2-dependent action of ghrelin on NPY/AgRP neurons is driven by a hypothalamic fatty acid oxidation pathway involving AMPK, CPT1 and free radicals that are scavenged by UCP2. These results reveal a signalling modality connecting mitochondria-mediated effects of G-protein-coupled receptors on neuronal function and associated behaviour.

Hypocretin (orexin) activation and synaptic innervation of the locus coeruleus noradrenergic system.

Hypocretin has been identified as a regulator of metabolic and endocrine systems. Several brain regions involved in the central regulation of autonomic and endocrine processes or attention are targets of extensive hypocretin projections. The most dense arborization of hypocretin axons in the brainstem was detected in the locus coeruleus (LC). Multiple labeling immunocytochemistry revealed a massive synaptic innervation of catecholaminergic LC cells by hypocretin axon terminals in rats and monkeys. In both species, all tyrosine hydroxylase‐immunopositive cells in the LC examined by electron microscopy were found to receive asymmetrical (excitatory) synaptic contacts from multiple axons containing hypocretin. In parallel electrophysiological studies with slices of rat brain, all LC cells showed excitatory responses to the hypocretin‐2 peptide. Hypocretin‐2 uniformly increased the frequency of action potentials in these cells, even in the presence of tetrodotoxin, indicating that receptors responding to hypocretin were expressed in LC neurons. Two mechanisms for the increased firing rate appeared to be a reduction in the slow component of the afterhyperpolarization (AHP) and a modest depolarization. Catecholamine systems in other parts of the brain, including those found in the medulla, zona incerta, substantia nigra or olfactory bulb, received significantly less hypocretin input. Comparative analysis of lateral hypothalamic input to the LC revealed that hypocretin‐containing axon terminals were substantially more abundant than those containing melanin‐concentrating hormone. The present results provide evidence for direct action of hypothalamic hypocretin cells on the LC noradrenergic system in rats and monkeys. Our observations suggest a signaling pathway via which signals acting on the lateral hypothalamus may influence the activity of the LC and thereby a variety of CNSfunctions related to noradrenergic innervation, including vigilance, attention, learning, and memory. Thus, the hypocretin innervation of the LC may serve to focus cognitive processes to compliment hypocretin‐mediated activation of autonomic centers already described. J. Comp. Neurol. 415:145–159, 1999.

Agouti-related peptide-expressing neurons are mandatory for feeding

Multiple hormones controlling energy homeostasis regulate the expression of neuropeptide Y (NPY) and agouti-related peptide (AgRP) in the arcuate nucleus of the hypothalamus. Nevertheless, inactivation of the genes encoding NPY and/or AgRP has no impact on food intake in mice. Here we demonstrate that induced selective ablation of AgRP-expressing neurons in adult mice results in acute reduction of feeding, demonstrating direct evidence for a critical role of these neurons in the regulation of energy homeostasis.

A Serotonin-Dependent Mechanism Explains the Leptin Regulation of Bone Mass, Appetite, and Energy Expenditure

Leptin inhibition of bone mass accrual requires the integrity of specific hypothalamic neurons but not expression of its receptor on these neurons. The same is true for its regulation of appetite and energy expenditure. This suggests that leptin acts elsewhere in the brain to achieve these three functions. We show here that brainstem-derived serotonin (BDS) favors bone mass accrual following its binding to Htr2c receptors on ventromedial hypothalamic neurons and appetite via Htr1a and 2b receptors on arcuate neurons. Leptin inhibits these functions and increases energy expenditure because it reduces serotonin synthesis and firing of serotonergic neurons. Accordingly, while abrogating BDS synthesis corrects the bone, appetite and energy expenditure phenotypes caused by leptin deficiency, inactivation of the leptin receptor in serotonergic neurons recapitulates them fully. This study modifies the map of leptin signaling in the brain and identifies a molecular basis for the common regulation of bone and energy metabolisms. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.